1. Field Of The Invention
The invention relates generally to currentsensing circuits in an industrial automation system and, more specifically, to a circuit for measuring FET or IGBT drain currents over a wide dynamic range.
2. Description Of The Relevant Art
Reduced to bare essentials, an industrial process may be regarded as having a number of sensors and loads that correspond to input and output variables for a process control computer system. The sensors provide input values representative of the state of the process at a given time; the loads respond to output values, and thereby control various aspects of the process. Typical sensors include relay contacts, proximity switches, and pressure switches. Typical loads include contractor coils of starters for large motors, solenoid valves, relays, lamps, and small motors. A process may have several hundred to several thousand input sensors and loads that must be serviced at very frequent intervals.
A typical computer system for automating an industrial process contains a number of general and special purpose computers. The system monitors input variables from the process, performs suitable logical manipulations on the inputs, and updates output variables for the process. The computer system is usually organized hierarchically. A host processor, typically a minicomputer or a mainframe, communicates with a number of programmable controllers, each of which communicates with a number of local processors and with a number of power control subsystems. A programmable controller is a processor designed specifically to perform logical manipulations on a large number of binary inputs on a cyclical basis. The local pocessors have as their primary function the efficient transfer of data between the power control subsystems and the working memories of the programmable controllers. The power control subsystems provide the interface between the local processors and the various sensors and loads.
One of the parameters frequently monitored by the host processor is current flowing through various components within the system. For example, it is desirable to ensure that excessive current does not flow through any component of the system, because such current frequently indicates a short-circuit or overload condition, which could result in catastrophic failure of the system. Similarly, it is also desirable to ensure that at least a prescribed minimum amount of current is flowing through the system, because the absence of such a minimum amount of current indicates a possible open-wire condition in the system, i.e., some component is not receiving current, or the component is broken, and therefore no current flows through it. It is equally important to detect this condition because automated systems require mechanical components to interact with each other, typically in an interleaving fashion, and if one component is not properly located at a particular time, collision of components may result with equally catastrophic consequences.
Modern industrial automation systems frequently use field-effect transistors (FET's) and insulated gate bipolar transistors (IGBT's) as current switching devices. FIG. 1A shows one such device FET's used in industrial automation systems frequently contain a gate terminal (G), a drain terminal (D), a mirror terminal (M), and a source terminal (S). Current flowing through the mirror terminal (M) ordinarily is a fraction of the overall current flowing through the device, and therefore it is this terminal which frequently is used to measure the current flowing through the device. This is done by placing a sense resistance R.sub.S between the mirror terminal and a ground potential, and then using a comparator to compare the voltage at a node (N) to a reference voltage.
The selected value for sense resistance R.sub.S depends upon the characteristics of the power device. FIG. 1b is a schematic representation of a circuit equivalent to the FET shown in FIG. 1a. As shown therein, the device equivalent circuit comprises a resistance R.sub.D, which is the overall resistance between the drain and source of the device. A typical value for R.sub.D is approximately 0.5 ohm. In parallel with R.sub.D is a resistance R.sub.M, which is the resistance of the device from the mirror terminal to the drain terminal. A typical value for R.sub.M is approximately 500 ohms. Because of such resistance values, conventional wisdom dictates that R.sub.S should be made as small as possible to measure the mirror current accurately. However, one cannot accurately measure small currents with a small senseresistance because the mirror voltage becomes impractically low (e.g., 50 mV or less). This is especially true when currents below 50 mA (a typical open-wire threshold current) are to be measured. Increasing the size of the sense resistance is not desirable, because the sense resistance R.sub.S then would contribute far more to the current measurement, and hence render the measurement highly inaccurate, if not useless.
One possible technique which attempts to overcome the problems of using large sense resistances, not necessarily in the prior art, is to measure the current flowing through the drain and source terminals of the device as a function of the external drain voltage of the device. However, FIG. 2 shows a graph of IGBT current as a function of external drain voltage and, as shown therein, the zero point of drain current remains at zero for external drain-source voltages greater than zero, and the relationship between the zero point of the external drain voltage and the zero point of the drain current must be known in order to measure accurately. However, the zero point of the external drain voltage is difficult to locate accurately.